Injection systems having a fuel accumulator and which are known as common rail injection systems have two actuators for pressure control, flow control being carried out by a metering unit (ZME) and pressure control via a pressure control valve (DRV). The pressure control valve may be operated strictly on a control basis in pressure control mode via the metering unit. Control of the pressure control valve is implemented in the pressure control mode, in such a way that the pressure control valve remains closed while taking into account all relevant sources of tolerance. If this is not ensured, then an unacceptable heating of the injection system and the fuel and the resulting increase in fuel consumption may be expected due to the resulting permanent leakage at the pressure control valve. High leakage may be detected by monitoring functions and then an emergency operation may be implemented.
In addition, the required reserve of tolerance at the pressure control valve may result in an injection system being operated with an excessively high opening pressure at the pressure control valve, depending on the tolerance situation. In the event of a defect, such as a stuck open metering unit, this may have the adverse effect that pressures prevail which are far greater than a nominal pressure of the injection system. The pressures occurring in such a fault case must not cause a failure of the injection system. If the pressures are too high, lines may rupture and cause fuel to escape into the engine compartment and result in a reduced stability or even failure of components of the injection system. Furthermore, excessively high pressures may result in loss of emergency driving ability to move a vehicle out of a danger zone. Taking into account a typical fault detection and response time, the components of the injection system must therefore be configured to be accordingly robust, which is complex and expensive.
So far, different adaptation functions are known for pressure control valves.
In one type of these adaptation functions for a pressure control valve, learning occurs only in pressure-controlled operation. Adaptation takes place in almost arbitrary or only slightly controllable boundary conditions with regard to a pressure and flow at the pressure control valve. A flow of fuel through the pressure control valve to be adapted is here always much greater than 0 L/h, which has a negative effect on the tolerance indication for the opening pressure of the pressure control valve. The highest pressure at which learning is typically possible with these functions is far below the pressure limit of the injection system. However, starting from this learned pressure, the tolerance indication deteriorates with an increase in pressure.
As an additional adaptation function, there is a known method in which the opening flow is ascertained by evaluating a rail pressure signal and/or a controller signal while the control current of the pressure control valve is varied.
The publication DE 10 2009 045 563 A1 discusses a method for determining at least one rail pressure closing current value pair for a pressure control valve of a common rail injection system of an internal combustion engine. In this method, the common rail injection system is operated in a metering unit-controlled mode, in which the trigger current for the pressure control valve is lowered, the pressure characteristic in the common rail over time is detected and the rail pressure is determined. The closing current is determined on the basis of the pressure characteristic thereby detected. Furthermore, a rail pressure closing current value pair is formed from the determined rail pressure and the determined closing current.
A method for operating a fuel system of an internal combustion engine is described in the publication DE 10 2004 059 330 A1, where a flow through a pressure control valve is taken into account as the basis for an adaptation function, the pressure control valve through which fuel may be discharged from a fuel pressure accumulator is precontrolled by a precontrol signal, which is ascertained by taking into account a setpoint pressure in the fuel pressure accumulator. A value for a quantity of fuel flowing through the pressure control unit is taken into account in ascertaining the precontrol signal.
Furthermore, additional publications for determining the operating parameters of injection systems are also known. Thus an individual pump characteristic line is learned in one method for operating an internal combustion engine using an injection system, which is discussed in the publication DE 10 2004 006 694 A1. In one method for operating a fuel injection system, which is discussed in the publication DE 10 2004 049 812 A1, an adaptation based on engine characteristics maps is carried out to balance out manufacturing tolerances in regulation of an operating parameter via a metering unit.